This invention relates in general to plasma arc cutting of sheet metals. More specifically, it relates to a mixture of type and proportion of gases forming and/or shielding the arc that yield very clean, shiny, and dross-free cuts in stainless steel, aluminum and other non-ferrous metals.
Plasma arc cutting of sheet metals is now used widely. However, heretofore for stainless steel and non-ferrous metals such as aluminum it has not been possible to produce a clean cut, one where there is a shiny kerf that is free of oxides or nitrides of the metal being cut, which is also free of bottom dross.
The plasma arc is a jet of an ionized gas. While many gases can be used to form the arc, the gas selected is usually specific to the metal being cut. For example, to cut stainless steel, it is most common to use air, nitrogen, or a mixture of argon and hydrogen.
Nitrogen and air leave no bottom dross, but the cut quality is poor. The sides of the kerf have oxide or nitride inclusions and they undergo a change in metallurgical structure. In order to weld at this cut, or to obtain an acceptable appearance, it is necessary to grind or wire-brush the cut sides.
It is known that argon-hydrogen may be used as the plasma gas to cut stainless steel. While these cuts are metallurgically "sheen", that is, shiny and clean, but at least for cuts in thin sheets, argon-hydrogen leaves a bottom dross that is unusually difficult to remove. Sheeny, dross-free cuts are possible with argon-hydrogen for sheets with a thickness in excess of about 0.5 inch (12.7 mm) using a 200 ampere torch and in excess of about 0.25 inch (6.4 mm) using a 100 ampere torch. No plasma cutting technique has been found that produces sheeny kerfs without dross when cutting aluminum, regardless of its thickness.
It is also well known to use shield gases, typically a secondary gas flow through the torch that is independent of the plasma gas flow and surrounds the arc, whether by impinging on it as it exits the torch or downstream, near or at the workpiece. Shield gases can serve a variety of functions, such as cooling, isolation of the cutting action in the kerf from the atmosphere, and the protection of the torch against upwardly splatterd molten metal. Plasma and shield gases are used, for example, in the plasma arc cutting torches sold by Hypertherm, Inc. of Hanover, N.H. under its trade designations MAX.RTM.200, MAX.RTM.100, MAX.RTM.100D and HD1070. The numbers 200, 100 and 70 denote current ratings for these torches. None of the known torches using shield gases have demonstrated any ability to improve on the cut quality of known nitrogen, air and argon-hydrogen cutting when used on stainless steel and non-ferrous metals such as aluminum.
It is therefore a principal object of the present invention to provide a plasma arc cutting process that can cut stainless steel, aluminum and other non-ferrous metals with an extremely high cut quality.
A further principal object is to provide a cutting process that is adaptable to different metals and different torches, including high density torches, and torches using only a plasma gas or ones using plasma and shield gases.
Another object is to provide a cutting process with the foregoing advantages even when used on thin sheets of the metal.
Still another object is to provide all of the foregoing advantages using known equipment and operating materials and at a favorable cost.